Double-fed asynchronous machines in the power range from 20 MVA to 500 MVA can be used for variable-speed energy production. These machines are distinguished by a distributed three-phase winding on the rotor. The rotor winding consists of individual bars which are embedded in slots in the rotor laminations. The individual bars are connected in the winding head to form a winding. The arrangement of the bar connections is uniformly distributed around the circumference. As a result of the rotation of the rotor, the winding heads are subjected to centrifugal forces, against which they have to be mechanically secured by means of winding head retention systems. In principle, three types of winding head retention systems can be used:    1. Fixing by means of a steel cap, as is the case with turbogenerators.    2. Fixing wherein a steel cable, wire or plastic film is wrapped around the whole winding head.    3. Fixing by means of bolts, screws or U-shaped brackets.
Such an asynchronous machine 10 is reproduced in section in highly simplified form in FIG. 1. It comprises a rotor 19 which can be rotated about an axis 18 and is encompassed concentrically by a stator with a stator lamination stack 14 with corresponding stator winding and a stator winding head 17. The rotor 19 comprises a central body 11 which merges with a shaft 11′ at each end. The central body 11 is surrounded by a rotor lamination stack 12 in which the rotor winding 13 runs. Slip rings 15, which are used to supply the rotor winding 13 with current, are arranged on one of the shafts 11′. A cylindrical air gap 21, which extends through the machine in the axial direction, is provided between the rotor lamination stack 12 and the stator lamination stack 14.
The air gap can have a different geometry in different operating phases depending on temperature, speed and other variable parameters. These changes do not substantially affect the function of the machine. However, it is also possible for the geometry of the rotor to permanently change in an intolerable manner or for debris to get into the air gap. In order to prevent consequential functional faults and to subject the machine to an inspection as soon as possible, it is necessary to detect changes of this kind in the air gap in good time.